The present invention relates to a method for preparing novel thermoset or thermosettable reticulate resin structures. It particularly relates to reticulate carbon structures derived from the resin structures.
U.S. Pat. No. 3,927,186 to Vinton and Franklin shows a method for producing reticulate carbon structures by rapid firing of reticulate polyurethane structures which have been infused with a liquid furan resin or resin precursor. U.S. Pat. No. 3,922,334 to Marek shows phenolic resin containing structures prepared by infusion of a reticulate polyurethane structure with a thermosettable phenolic resin dissolved in tetrahydrofuran as a solvent and then carbonization for a longer period. These methods work very well but after the infusion step considerable care is required to remove unalloyed or non-infused resin from the surfaces of the polyurethene reticulate structure. The liquid thermosettable resins or resin precursors require an infusion step followed by an excess removal step, both entailing handling operations. It would be preferred if the thermoset resin structures could be produced directly by foaming and then reticulated so that the infusion and excess removal steps could be avoided.
The use of a combustible gas mixture to reticulate polyurethane foams, particularly flexible foams as well as other cellular materials, is described in U.S. Pat. No. 3,175,025. The method is used commercially world-wide for reticulating flexible polyurethane foams by destroying the cell membranes; however, it has not been commercially used for other types of foams. The polyurethane foams have a specific foam structure with thin relatively uniform cross-sectioned cell membranes attached to much thicker strands defining the cells and also defining the intersection of membranes which are most susceptible to thermal reticulation. The cell membranes join the strands with a relatively very small radius of curvature between them. As a result the heat from the combustion, even though transient, destroys the membranes. On the other hand, thermoset resins usually do not form polyurethane-like foams, but rather the cells tend to be spherical with relatively irregularly cross-sectioned, usually closed cellular partitions between the cells. There is a relatively poor or non-existent definition between strands and membranes in the cells. The thermoset resin foams of U.S. Pat. No. 3,121,050 to Ford are generally believed to be of this type.
The prior art thermoset resin foams have the advantage of being converted to carbon rather than being volatilized during heating. Polyurethane foams which are flexible volatilize virtually completely when heated above about 400.degree. C. and do not carbonize. Rigid polyurethane foams which are more cross-linked will carbonize, as shown by U.S. Pat. No. 3,302,999 to Mitchell, and are essentially closed cell (unicellular) foams.
U.S. Pat. Nos. 3,345,440 to Googin et al.; 3,574,548 to Sands et al.; 3,635,676 to Sands et al.; 3,857,913 to Crow et al.; and 3,975,318 to Larsen et al. show unreticulated thermoset resin membrane-containing foams some of which are described as being carbonized over relatively long periods of time and wherein the carbon product contains essentially all of the membranes present in the uncarbonized foam. It would be desirable to be able to reduce the carbonization period and preferably at the same time produce reticulated carbon structures without having to infuse a polyurethane reticulate structure as in U.S. Pat. No. 3,972,186 to Vinton et al.